BUG: fix error messages in RAMSESDataset validation

[neutrinoceros]

PR Summary

This adresses the surface problem reported in #3800, i.e., I'm demystifying errors raised by RAMSESDataset at validation time

Example 1

from yt.frontends.ramses.api import RAMSESDataset

RAMSESDataset("not_a_file")

On main

ValueError: ('Invalid filename found when building a RAMSESDataset object: %s', 'not_a_file')

This branch

FileNotFoundError: No such file or directory '/Users/robcleme/dev/yt-project/yt/not_a_file'

Example 2

import os
from yt.frontends.ramses.api import RAMSESDataset

RAMSESDataset(os.path.expanduser("~"))

On main

ValueError: ('Invalid filename found when building a RAMSESDataset object: %s', '/Users/robcleme')

This branch

ValueError: Could not determine output directory from '/Users/robcleme'
Expected a directory name of form '(output|group)_(\\d{5})'

[matthewturk]

Good improvement.

[neutrinoceros]

@matthewturk sorry I just forced pushed again because I realised I made a mistake that could break yt.load(None)

[neutrinoceros]

This took a surprising amount of iterations for me to understand enough of what the frontend actually expected so I can provide more useful error messages. So I think the problem Andrew actually had was that his data isn't stored in a dir that's named output_xxxxx or group_xxxxxx. I've now made this expectation much more apparent in the error message :

ValueError: Could not determine output directory from '/Users/robcleme'
Expected a directory name of form '(output|group)_(\\d{5})'

I'll wait on his reply to see if he's indeed able to load his data after rearranging the files, but I expect that this is now sufficient to close the issue.

[cphyc]

LGTM

[cphyc]

Ooops, I should have waited a bit more before merging, shouldn't I?

[neutrinoceros]

I don't think so, why ?

[neutrinoceros]

(If it turns out that the issue Andrew had was more complex I can always reopen the ticket)